This invention concerns radio frequency ID tags, preferably passive RFID tags in miniature size, and a system for efficiently powering and reading the tags to track or inventory products carrying the tags.
The concept of tracking, counting or inventorying items of commerce using radio frequency ID (RFID) tags is well known. See, for example, U.S. Pat. No. 6,480,699, describing miniaturized passive RFID tags with onboard antennas. These tags can be as small as about 0.5 mm×0.5 mm with an even smaller thickness dimension, and they are powered solely by a powering RF signal sent by a reader or power module, thus storing sufficient power for a sufficient period of time to transmit data carried by the particular RFID tag.
RFID interrogators or readers, which transmit a relatively high-powered signal in order to power up the tags within a coverage area, are costly and their reach for powering the tags is limited. A tag which has been powered, (especially a tag as in the above patent and as in copending application Ser. No. 10/919,800 of Tagent Corporation) will exhibit an effective transmit distance far greater than the reach of a powering RF signal that is within the range of size, cost and power practical for use in RFID applications. In other words, in a typical situation the transmitted signal from each RFID tag can be read from a far greater distance than the distance to which an interrogator device sending out a powering signal can reach to power up the tags. As a result, in the usual situation the tags (and the attached products) must be brought close to the reader to be accounted for, inventoried and tracked, unless multiple costly readers are dispersed throughout an area.